A preferred field of application of the present invention is the serial production of technical products, such as motor vehicles. Frequently, a unit of the technical product is produced in response to a customer order. The order may include several units. A delivery date is agreed with the customer, the delivery date being the date on which the unit of the technical product ordered by the customer is made available. A final inspection date on which the unit is finished is derived from this delivery date. This final inspection date depends on the actual cycle time of the product through the serial production process. In serial production, the actual cycle time through the production process varies as well, for example, due to differences in quality of supplied parts and because of breakdowns and other unpredictable events. In practice, therefore, it may happen that a final inspection data, and thus, a delivery date for a particular unit of the technical product cannot be met because the actual cycle time is longer than the planned cycle time that formed the basis for the agreed final inspection date. This planned cycle time is often also referred to as the maximum permitted cycle time for a production process. Delivery reliability (on-time delivery) means that a predetermined final inspection date is met. In the following, an order will be referred to as completed on time even if the unit is completed earlier than agreed. The proportion of units of the technical product whose actual cycle times are less than or equal to a specified planned cycle time and whose final inspection dates may therefore be met is referred to as the degree of delivery reliability. In the prior art, the degree of delivery reliability is sometimes also referred to as the degree of on-time delivery.
A serial production process includes a plurality of subprocesses, such as several trades in a vehicle factory. The cycle time through the entire production process depends on the cycle times through the subprocesses. When defining maximum cycle times for the subprocesses, two different objectives must be taken into account:                The highest possible degree of delivery reliability should be achieved since late delivery of units of the technical product may result in penalties.        The average storage duration for units of the technical product that are completed prior to the final inspection date should be as short as possible since storage binds capital, requires space for the units, and involves the risk of damage to finished units during storage. On the other hand, units that are completed exactly on the final inspection date or later do not require storage.        
The two objectives are in conflict with each other because the longer the planned cycle time through the production process the greater is the degree of delivery reliability, but the shorter the cycle time the shorter is the average storage duration.
U.S. Pat. No. 6,259,959 B1 discloses a method and device for determining the impact of components, such as workstations, of a manufacturing line on the performance of the manufacturing line. The X-factor of a component, which is the quotient of the cycle time divided by the raw processing time, is used as a measure of performance. The cycle time is the period that elapses between the instant at which a workpiece reaches the component and the instant at which it leaves the component. The X-factor factor increases significantly with increasing throughput through the manufacturing line. To achieve a given cycle time, it is often necessary to reduce the throughput, which involves costs. Therefore, a compromise must be found between throughput, degree of delivery reliability, capacity, and high utilization of the components. The X-factors and, in one embodiment, the throughput through the production process are used to derive assessments of the components. Optimization measures are directed to components having a low rating.
The method and device of U.S. Pat. No. 6,259,959 B1 require that the raw processing time of each subprocess be measured or otherwise determined. Such results of the measurement of the raw processing time, which is only part of the total cycle time through a subprocess, are frequently not available. Moreover, it is not described how a certain degree of delivery reliability can be maintained.
U.S. Pat. No. 6,195,590 B1 discloses a method and device for monitoring compliance with a schedule for a production process including a plurality of subprocesses. There are specified a desired completion date of the production process, such as a final inspection date, as well as estimated cycle times through the subprocesses, and availabilities of external events that must have occurred for subprocesses to begin. Based on the completion date and the cycles times, desired starting times (“baseline schedule data”) are derived for the subprocesses and compared to the target dates of external events. The greatest deviation is determined as well as the subprocess responsible for it. This document does not describe either how a certain degree of delivery reliability can be maintained.
Known from U.S. Pat. No. 5,229,948 is a method of optimizing a serial production process including a plurality of subprocesses (“stages”). A quantitative model of states, implemented, for example, by buffer memories, is established, and the performance of the production process and individual subprocesses is determined by model simulations. When necessary, it is determined which buffer memories need to the changed in order to produce the greatest improvement. The establishment and adaptation of such a model requires considerable effort and is prone to errors.